Know about 3 Important Ways for DC Motor Speed Control

DC motor speed control is one of the most useful features of the motor. By controlling the speed of the motor, you can vary the speed of the motor according to the requirements and can get the required operation.

DC Motor Speed Control

The speed control mechanism is applicable in many cases like controlling the movement of robotic vehicles, movement of motors in paper mills and the movement of motors in elevators where different types of DC motors are used.

DC Motor’s Working Principle

A simple DC motor works on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force. In a practical DC motor, the armature is the current carrying the conductor, and the field

DC Motor working principle

provides magnetic field.

When the conductor (armature) is supplied with a current, it produces its own magnetic flux. The magnetic flux either adds up to the magnetic flux due to the field windings at one direction, or cancels the magnetic flux due to field windings. The accumulation of magnetic flux at one direction compared to the other exerts a force on the conductor, and therefore, it starts rotating.

According to Faraday’s law of electromagnetic induction, the rotating action of the conductor produces an EMF. This EMF, according to Lenz’ law, tends to oppose the cause, i.e., the supplied voltage. Thus, a DC motor has a very special characteristic of adjusting its torque in case of varying load due to the back EMF.

The Principle of Speed Control

From the above figure, the voltage equation of a simple DC motor is
V = Eb + IaRa
V is the supplied voltage, Eb is the back EMF, Ia is the armature current, and Ra is the armature resistance.
We already know that
Eb = (PøNZ)/60A.
P – number of poles,
A – constant
Z – number of conductors
N- speed of the motor
Substituting the value of Eb in the voltage equation, we get
V = ((PøNZ)/60A) + IaRa
Or, V – IaRa = (PøNZ)/60A
i.e., N = (PZ/60A) (V – IaRa)/ ø
The above equation can also be written as:
N = K (V – IaRa)/ ø, K is a constant

This implies three things:

Speed of the motor is directly proportional to supply voltage.

Speed of the motor is inversely proportional to armature voltage drop.

Speed of the motor is inversely proportional to the flux due to the field findings

Thus, the speed of a DC motor can be controlled in three ways:

By varying the supply voltage

By varying the flux, and by varying the current through field winding

By varying the armature voltage, and by varying the armature resistance

3 Ways of DC Motor Speed Control

1. Flux Control Method

In this method, the magnetic flux due to the field windings is varied in order to vary the speed of the motor.

Flux Control method

As the magnetic flux depends on the current flowing through the field winding, it can be varied by varying the current through the field winding. This can be achieved by using a variable resistor in a series with the field winding resistor.

Initially, when the variable resistor is kept at its minimum position, the rated current flows through the field winding due to a rated supply voltage, and as a result, the speed is kept normal. When the resistance is increased gradually, the current through the field winding decreases. This in turn decreases the flux produced. Thus, the speed of the motor increases beyond its normal value.

2. Armature Control Method

With this method, the speed of the DC motor can be controlled by controlling the armature resistance to control the voltage drop across the armature. This method also uses a variable resistor in series with the armature.

Armature Control method

When the variable resistor reaches its minimum value, the armature resistance is at normal one, and therefore, the armature voltage drops. When the resistance value is gradually increased, the voltage across the armature decreases. This in turn leads to decrease in the speed of the motor.

This method achieves the speed of the motor below its normal range.

3. Voltage Control Method

Both the above mentioned methods cannot provide speed control in the desirable range. Moreover, the flux control method can affect commutation, whereas the armature control method involves huge power loss due to its usage of resistor in series with the armature. Therefore, a different method is often desirable – the one that controls the supply voltage to control the motor speed.

In such a method, the field winding receives a fixed voltage, and the armature gets a variable voltage.
One such technique of voltage control method involves the use of a switch gear mechanism to provide a variable voltage to the armature, and the other one uses an AC motor driven Generator to provide variable voltage to the armature (the Ward-Leonard System).

Apart from these two techniques, the most widely used technique is the use of pulse width modulation to achieve speed control of a DC motor. PWM involves application of varying width pulses to the motor driver to control the voltage applied to the motor. This method proves to be very efficient as the power loss is kept at minimum, and it doesn’t involve the use of any complex equipment.

Voltage Control Method

The above block diagram represents a simple electric motor speed controller. As depicted in the above block diagram, a microcontroller is used to feed PWM signals to the motor driver. The motor driver is L293D IC which consists of H-bridge circuits to drive the motor.

PWM is achieved by varying the pulses applied to the enable pin of the motor driver IC to control the applied voltage of the motor. The variation of pulses is done by the microcontroller, with the input signal from the push buttons. Here, two push buttons are provided, each for decreasing and increasing the duty cycle of pulses.

We hope that we have been able to provide a detailed and relevant description on DC motor speed control. Here is a simple question for our readers: What are the other methods of applied voltage control apart from PWM?

my project proposal is to build is to used a dc motor to turn a gear of two or three local or simple grinding machine for groundnuts.that was achieve by turning the handle manually.
please can you help me with some ideas

Hi Amadou jallow, Sorry we do not have your project. Please check the user friendly website http://www.efxkits.com to see if anything near your requirements can be met. Please check the website for complete details in the kit content section: http://www.efxkits.com/kitdetails/ and go through the FAQ: http://www.efxkits.com/contact-us/#faq for details. Further if you have any queries you can mail us at info@edgefxkits.in or please call on +91 9908778000 for clarifications/personal counseling. Raji (Contact India: +919959178000, Contact International: +1 201 44 83073)

thanks from your information.but I need some for triystor &inverteri control of big motor.for example in water stations for transfer pipeline from 5mw up to10 mw & bigger with 6.6 kv.
thank you so much for me.